In the starting room of the chapter, there is a hole in the wall in front of you. A grenade needs to be thrown into the hole. One of those can be obtained by quickly killing an enemy thug who attempts to throw it later in the level. After the explosive detonates in the hole, an objective titled "I had declared a war against rats" will be added along with the main objective.
Continue along with the level until you get to the stair platform, close to where the unseen bomb detonates and shakes the walls. There, if you get close enough to the rats running around, they will equip themselves with either a Beretta or a Desert Eagle, and will start firing at you. The rats are pretty difficult to kill due to their small size. At times, the rats shoot at each other.